Synthesis and Biological Evaluation of Novel Paclitaxel (Taxol) D-Ring

For example, treatment of 11 with Ac2O/DMAP in CH2Cl2 or LHMDS/AcCl, or the ... THF, −20 to 25 °C, 2 h, 67%; (ii) LHMDS, THF, −78 °C (5 min), rt...
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J. Org. Chem. 1999, 64, 2694-2703

Synthesis and Biological Evaluation of Novel Paclitaxel (Taxol) D-Ring Modified Analogues A. A. Leslie Gunatilaka,† Frank D. Ramdayal, Maria H. Sarragiotto,‡ and David G. I. Kingston* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0212

Dan L. Sackett and Ernest Hamel Laboratory of Drug Discovery Research and Development, Developmental Therapeutics Program, Division of Cancer Treatment and Diagnosis, National Cancer Institute, Frederick Cancer Research and Development Center, Frederick, Maryland 21702 Received October 19, 1998

The semisynthesis and biological activity of paclitaxel (Taxol) analogues in which the oxygen atom in ring D is substituted by a sulfur or a selenium atom is presented. These derivatives were synthesized and tested in order to make more transparent the role of the oxetane ring in the biological activity of paclitaxel. The sulfur derivatives were found to be less active than paclitaxel in biological assays, while the selenium derivative could not be converted to its 4-acyl analogue. The results with the sulfur analogues suggest that the oxygen atom in the oxetane ring plays an important role in the mechanism by which paclitaxel exhibits its anticancer activity. Introduction The anticancer drug paclitaxel (Taxol, 1a), a diterpenoid isolated from the bark of Taxus brevifolia, is clinically used in the treatment of ovarian and breast cancer.1 Since its discovery in the late 1960s,2 intensive studies of its chemistry and structure-activity relationships have been reported.3 These studies have established that the C-13 side chain, the ester groups at C-2 and C-4, and the oxetane ring are all essential for biological activity,3 but the precise role of these groups in paclitaxel’s activity remains obscure. It is known that paclitaxel binds to microtubules, thereby stabilizing them and disrupting the equilibrium between tubulin and microtubules, and thus halting cell division,4 but the conformation in which paclitaxel binds to microtubules and the actual binding site are not known * To whom correspondence should be addressed. Phone: 540-2316570. Fax: 540-231-7702. E-mail: [email protected]. † Present address: Bioresources Research Facility, University of Arizona, Tucson, AZ 85706-6800. ‡ Visiting faculty from Department of Chemistry, Universidade Estadual de Maringa, Maringa, PR-Brazil. (1) (a) Holmes, F. A.; Kudelka, A. P.; Karanagh, J. J.; Huber, M. H.; Ajani, J. A.; Valero, V. In Taxane Anticancer Agents: Basic Science and Current Status; Georg, G. I., Chen, T. T., Ojima, I.; Vyas, D. M., Eds.; ACS Symposium Series 583, American Chemical Society: Washington, DC, 1994; pp 31-57. (b) Arbuck, S. G.; Blaylock, B. A. In Taxol: Science and Applications; Suffness, M., Ed.; CRC Press Inc.: Boca Raton, FL, 1995; pp 379-415. (c) McGuire, W. P., Rowinsky, E. K., Eds. Paclitaxel in Cancer Treatment; Marcel Dekker: New York, Basel, Hong Kong, 1995; Vol. 8, pp 1-349. (2) Wani, M. C.; Taylor, H. L.; Wall, M. E.; Coggon, P.; McPhail, A. T. J. Am. Chem. Soc. 1971, 93, 2325-2327. (3) For recent reviews: (a) Kingston, D. G. I.; Molinero, A. A.; Rimoldi, J. M. Prog. Chem. Org. Nat. Prod. 1993, 61, 1-188. (b) Suffness, M. Annu. Rep. Med. Chem. 1993, 28, 305-314. (c) Georg, G. I.; Ali, S. M.; Zymunt, J.; Jayasinghe, J. R. Expert Opin. Ther. Pat. 1994, 12, 222-227. (d) Chen, S. H.; Farina, V. Paclitaxel (Taxol) Chemistry and Structure Activity Relationships. In The Chemistry and Pharmacology of Taxol and its Derivatives; Farina, V., Ed.; Elsevier: New York, 1995; pp 165-223. (e) Kingston, D. G. I. Trends Biotechnol. 1994, 12, 222-227. (4) Horwitz, S. B. Trends Pharmacol. Sci. 1992, 13, 134-136.

in detail. As far as the conformation of paclitaxel is concerned, NMR studies in aqueous solution indicate that a “hydrophobic collapse” conformation, resulting from hydrophobic clustering of the C-2 benzoyl, C-3′ phenyl, and C-4 acetate groups, is probably important for providing the critical side chain conformation relevant to the binding of paclitaxel to microtubules.5 These findings provide at least a partial explanation for the importance of the groups indicated but do not provide any clue to the importance of the oxetane ring. It is possible to devise various hypotheses for the importance of the oxetane ring to paclitaxel’s activity. Thus it might cause the acetoxy group to adopt an orientation in space favorable for interaction with microtubules by increasing the rigidity of ring C (conformational role) or it might be directly involved in receptor binding by its oxygen atom through hydrogen bonding (electronic role). Thus the effect of replacement of the oxygen atom in the oxetane ring by another heteroatom on the activity of paclitaxel might clarify whether the role (5) (a) Vander Velde, D. G.; George, G. I.; Grunewald, G. L.; Gunn, C. W.; Mitscher, L. A. J. Am. Chem. Soc. 1993, 115, 11650-11651. (b) Dubois, J.; Guenard, D.; Gueritte-Voegelein, F.; Guerida, N.; Potier, P.; Gillet, B.; Beloeil, J. C. Tetrahedron 1993, 49, 6533-6544. (c) Williams, H. J.; Scott, A. I.; Dieden, R. A.; Swindell, C. S.; Chirlian, L. E.; France, M. M.; Heerding, J. M.; Kraus, N. E. Tetrahedron 1993, 49, 6545-6560. (d) Paloma, L. G.; Guy, R. K.; Wrasidio, W.; Nicolaou, K. C. Chem. Biol. 1994, 1, 107-112.

10.1021/jo982095h CCC: $18.00 © 1999 American Chemical Society Published on Web 03/30/1999

Novel Paclitaxol D-Ring Modified Analogues

J. Org. Chem., Vol. 64, No. 8, 1999 2695 Scheme 1

of the oxetane ring is primarily conformational or primarily electronic. Recently, the syntheses of the azetidine derivatives 2a and 2b6 and 3a and 3b7 were reported. The derivatives

2a and 2b lacked the benzoyloxy function at both the C-2 and the C-13 side chain, both of which are known to be important for biological activity,3 and thus did not provide an appropriate model to test the importance of the oxetane ring. The azetidine analogues 3a and 3b were both inactive in an in vitro cytotoxicity assay; analogue 3a did show some tubulin polymerization activity, but this was 16 times less than that of docetaxel (1b).7 These results were explained in terms of a specific interaction of the heteroatom with the protein. The presence of a nitrogen atom in the azetidine ring also posed the problem of basicity, in that it would be protonated at neutral pH and thus would presumably interact with tubulin in a very different way than a neutral oxygen atom. For these reasons, the role of the oxetane ring on the activity of paclitaxel was still unclear, and the synthesis of other D-ring modified analogues of paclitaxel was necessary. Replacement of the oxygen atom with a sulfur or a selenium atom would maintain the neutrality of the ring while at the same time change both steric and electronic effects in a predictable way. We thus decided to embark on the synthesis of the 5,20-thiapaclitaxel analogue 16 and the 5,20-selenapaclitaxel analogue 24. Results and Discussion Chemistry. Our strategy for the synthesis of 5,20thiapaclitaxel analogues was envisaged to involve recon(6) Fenoglio, I.; Nano, G. M.; Vander Velde, D. G.; Appendino, G. Tetrahedron Lett. 1996, 37, 3202-3206. (7) Marder-Karsenti, R.; Dubois, J.; Bricard, L.; Gue´nard, D.; Gue´ritte-Voegelein, F. J. Org. Chem. 1997, 62, 6631-6637.

struction of the D-ring from a C-5R-halo-4,20-oxirane intermediate with simultaneous introduction of the heteroatom (Scheme 1). The C-5R leaving group could be introduced from the oxetane ring opening, and the 4,20epoxy group would be formed from cyclization of the C-4hydroxy group onto the appropriately functionalized C-20 hydroxyl group. Protection of C-1 and C-2 hydroxy groups was required to avoid the formation of undesired products in the oxetane ring opening step;8 the protection of these groups as a cyclic carbonate was expected to facilitate acylation of C-4 and also to create conditions for the introduction of the benzoate functionality at C-2.9 The synthetic approach we have developed to the 4,20-oxirane intermediate 10 is shown in Scheme 2. The starting material, paclitaxel (1a), was protected at the C-2′ and C-7 positions by tert-butyldimethylsilyl and triethysilyl groups, respectively, to give the protected derivative 4 in 91% yield.10 Selective hydrolysis of the C-2 benzoate and C-4 acetate groups of 4 with Triton B afforded the triol 6 in 66% yield.10 Protection of the C-1 and C-2 hydroxy groups in 6 with carbonyldiimidazole and imidazole11 gave the 1,2-carbonate derivative 7 in 85% yield. Opening of the oxetane ring in the protected derivative 7 was first attempted with trimethylsilyl chloride and lithium iodide, but these conditions produced a mixture of compounds. After some experimentation, it was found that treatment of 7 at -78 °C in dichloromethane with iodotrimethylsilane followed by a careful workup afforded the 5R-iodo-20-hydroxy-D-seco-paclitaxel derivative 8 in 93% yield. When workup was effected at room temperature, the formation of the C-2′ desilylated compound 9 was also observed. Finally, the formation of the 5R-iodo20-epoxy derivative 10 was achieved in 96% yield by the reaction of 8 with trifluoromethanesulfonyl chloride and DMAP.

(8) Chen, S. H.; Huang, S.; Wei, J.; Farina, V. Tetrahedron 1993, 49, 2805-2828. (9) (a) Holton, R. A.; Somoza, C.; Kim, H. B. Liang, F.; Biediger, R. J.; Boatman, D.; Shindo, M.; Smith, C. C.; Kim, S.; Nadizadeh, H.; Suzuki, Y.; Tao, C.; Vu, P.; Tang, S.; Zhang, P.; Murthi, K. K.; Gentile, L. N.; Liu, J. H. J. Am. Chem. Soc. 1994, 116, 1597-1599. (b) Nicolaou, K. C.; Yang, Z.; Liu, J. J.; Ueno, H.; Nantermet, P. G.; Guy, R. K.; Claiborne, C. F.; Renaud, J.; Couladouros, E. A.; Paulvannan, K.; Sorensen, E.; J. Nature 1994, 367, 630-634. (c) Masters, J. J.; Linj, J. T.; Snyder, L. B.; Young, W. B.; Danishefsky, S. J. Angew. Chem., Int. Ed. Engl. 1995, 34, 1723-1726. (10) Chordia, M. D.; Kingston, D. G. I. J. Org. Chem. 1996, 61, 799801. (11) Nicolaou, K. C.; Renaud, J.; Nantermet, P. G.; Couladouros, E. A.; Guy, R. K.; Wrasidlo, W. J. Am. Chem. Soc. 1995, 117, 2409-2420.

2696 J. Org. Chem., Vol. 64, No. 8, 1999

Gunatilaka et al. Scheme 2a

a Key: (i) TBDMS/imidazole, DMF, 60 °C, 2 h, then Et SiCl/imiazole, 1 h, 91%; (ii) Triton B, CH Cl , -78 to -10 °C, TLC control, 66%; 3 2 2 (iii) carbonyldiimidazole, CH2Cl2, 35-40 °C, 24 h, 85%; (iv) Me3SiI, CH2Cl2, -78 °C, 45 min, 93%; (v) TfCl/DMAP, CH2Cl2, 0 °C to rt, 1 h, 96%.

Scheme 3a

a Key: (i) Li S, THF, rt, 28 h, then carbonyldiimidazole/imidazole, rt, 12 h, 11 (56%), 12 (13%); (ii) LHMDS, THF, -78 °C (7 min), rt 2 (1 min), -78 °C (2 min), then methyl chloroformate, -78 °C (10 min), 14 (39%), 11 (41%); (iii) PhLi, THF, -78 °C, 3 min, 61%; (iv) HF-pyridine, rt, 9 h, 76%; (v) m-CPBA, CH2Cl2, 0 °C to rt, 80 min, 61%.

Having made the key intermediate 10, we next turned our attention to the preparation of 5,20-thiapaclitaxel analogues; the synthetic route is summarized in Scheme 3. Treatment of the intermediate 10 with lithium sulfide in THF at room temperature, with monitoring of the reaction by TLC, showed that there was a complete disappearance of starting material after 28 h. The 1H NMR spectrum of the product obtained indicated it to be a 1,2,4-triol different from 6. Treatment of the crude triol reaction product with carbonyldiimidazole and imidazole afforded two products after purification, namely the sulfide 11 (56%) as the major product and the disulfide 12 (13%) as the minor product. When a very large excess of Li2S was used the major product from the

reaction of 10 with Li2S was the disulfide 12 (42%), and sulfide 11 (20%) became the minor product, so either 11 or 12 could be formed as the major product by controlling the concentration of Li2S. The mechanism for the formation of 11 presumably involves nucleophilic attack on the primary C-20 of the 4,20-epoxy group by the sulfide ion, followed by a ring closure step involving attack on C-5 and displacement of iodide;12 the alternative pathway of initial attack at C-5 is less likely on steric grounds. Thus, (12) A similar cyclization was used in Wender’s synthesis of taxol: Wender, P. A.; Badham, N. F.; Conway, S. P.; Floreancig, P. E.; Glass, T. E.; Houze, J. B.; Krauss, N. E.; Lee, D.; Marquess, D. G.; McGrane, P. L.; Meng, W.; Natchus, M. G.; Shuker, A. J.; Sutton, J. C.; Taylor, R. E. J. Am. Chem. Soc. 1997, 119, 2757-2758.

Novel Paclitaxol D-Ring Modified Analogues

J. Org. Chem., Vol. 64, No. 8, 1999 2697 Scheme 4a

a Key: (i) LHMDS, THF, -78 °C (7 min), rt (1 min), -78 °C (2 min), then methyl chloroformate, -78 °C (10 min), 69%; (ii) PhLi, THF, -78 °C, 3 min, 70%; (iii) HF-pyridine, rt, 9 h, 87%.

contrary to previous reports,13 nucleophilic opening of the 4,20-epoxide is possible under the conditions employed. The sulfide 11 could be oxidized with 3-chloroperbenzoic acid to the sulfone (13) in 61% yield. We next turned our attention to the subsequent steps of the synthesis, i.e acetylation of C-4, benzoylation of C-2 and deprotection of C-2′ and C-7 positions. Surprisingly, the C-4 hydroxy group in 11 was found to be resistant to several acylation conditions. For example, treatment of 11 with Ac2O/DMAP in CH2Cl2 or LHMDS/ AcCl, or the use of other acylating agents such as cyclopropyl carbonyl chloride or N-acetylimidazole, led only to the recovery of starting material. Attempted acetylation under forcing conditions with acetic anhydride and DMAP in toluene at 80 °C also failed to provide the acetylated product. After many attempts, it was found that treatment of 11 with LHMDS and methyl chloroformate afforded the derivative 14 in 39% yield, along with unreacted starting material (41%). Attempts to increase the yield by increasing the reaction time, temperature, or concentration of reagents gave a mixture of polar products. Benzoylation of the hydroxy group at C-2 in 14 was effected by reaction with phenyllithium, at -78 °C in THF,14 affording 15 in 61% yield. Hydrolysis of the methoxycarbonyl group and formation of a complex mixture of products was observed when workup was carried out by standard methods. Thus a modified workup, involving quenching of the reaction with dilute aqueous HCl solution, was employed to avoid the hydrolysis of the methoxycarbonyl group. Conversion of 15 to the 5,20-thiapaclitaxel analogue 16 was achieved in 76% yield by treatment with HF-pyridine at room temperature. The spectroscopic data for 16 were consistent with its assigned structure. Attempts to acetylate the C-4 hydroxy group of the disulfide 12 using the methods just described were also unsuccessful and led to the recovery of unreacted 12. Also, attempts to react 12 with methyl chloroformate under the conditions that were used successfully with the sulfide, 11, were also unsuccessful and led to the recovery of unreacted 12. Thus, it appears that the C-4 hydroxy group of 12 is very sterically hindered and hence very difficult to acylate. (13) (a) Holton, R. Total Synthesis of Paclitaxel from Camphor. In Taxane Anticancer Agents: Basic Science and Current Status; George, I., Chen, T. T., Ojima, I., Vyas, D. M., Eds.; ACS Symposium Series 583, American Chemical Society: Washington, DC, 1995; pp 288-301. (b) Margraff, R. M.; Bezard, D.; Bouzart, J. D.; Commerc¸ on, A. Bioorg. Med. Chem. Lett. 1994, 4, 233-236. (14) Holton, R. A.; Kim, H. B.; Somoza, C.; Liang, F.; Biendiger, R. J.; Boatman, P. D.; Shindo, M.; Smith, C. C.; Kim, S.; Nadizadeh, H.; Suzuki, Y.; Tao, C.; Yu, P.; Tang, S.; Zhang, P.; Murthi, K. K.; Gentile, L. N. Liu, J. H. J. Am. Chem. Soc. 1994, 116, 1599-1600.

The sulfide analogue 16 of paclitaxel contains two structural changes compared to paclitaxel, in that the oxygen atom in the oxetane ring of paclitaxel has been replaced by a sulfur atom and the C-4 acetate group has been replaced by a methoxycarbonyl group. Hence, to get a better approximation of the independent effect of each of these structural changes on biological activity, we synthesized the paclitaxel derivative 19 (Scheme 4), in which only the C-4 acetate in paclitaxel has been replaced by a methoxycarbonyl group. By doing this we were able to compare the biological activity of paclitaxel with 19 and then the biological activity of 19 with the sulfide 16. Thus, as shown in Scheme 4, treatment of the protected 1,2-carbonate derivative 7 with LHMDS and methyl chloroformate gave 17 (69% yield) substituted at the C-4 position by a methoxycarbonyl group. Benzoylation of 17 with phenyllithium gave the 4-methoxycarbonyl-2-benzoyl derivative 18 in 70% yield. Finally, removal of the C-2′ and C-7 silyl protecting groups in 18 with HFpyridine gave the required product 19 in 87% yield. The successful preparation of the 5,20-thiapaclitaxel analogue by reconstruction of the four-membered ring with the simultaneous introduction of a heteroatom from intermediate 10 led us to extend this strategy to the synthesis of the 5,20-selenapaclitaxel analogue 23 (Scheme 5). A similar replacement of an oxygen atom for a selenium atom could be achieved by reaction of key intermediate 10 with lithium selenide. As reported,15 organoselenium compounds can be prepared by reacting alkyl halides with lithium selenide, the latter being available from the reaction of Li(C2H5)3BH with gray selenium. Indeed, treatment of a THF solution of 10 with Li2Se prepared as above resulted in the formation of the 5,20selena derivative 20 in 67% yield. Regrettably we were unable to isolate the desired 4-acyl analogue 21 by acylation of 20. The C-4 hydroxy group of 20 was unreactive toward several acylating conditions, such as Ac2O/DMAP in CH2Cl2, LHMDS/AcCl in THF, acylimidazole/DMAP, or LHMDS and LHMDS/ cyclopropyl carbonyl chloride. Attempted acylation with LHMDS/methyl chloroformate, which had proven to be successful in the case of the thiapaclitaxel analogue 11, was unsuccessful in this case and afforded a reaction product that was tentatively identified as a mixture of the desired analogue 21 and the ring-opened derivative 25 (Scheme 5). The presumed compound 21 was however unstable and was converted to 25 either on attempted (15) Hopkins, P. B.; Fuchs, P. L. J. Org. Chem. 1978, 43, 12041208.

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Gunatilaka et al. Scheme 5a

a Key: (i) Li Se, THF, -20 to 25 °C, 2 h, 67%; (ii) LHMDS, THF, -78 °C (5 min), rt (1 min), then methyl chloroformate, -78 °C (5 min), 2 25 (48%), 20 (14%); (iii) PhLi, THF, -78 °C, 3 min, 43%; (iv) HF-pyridine, rt, 8 h, 70.

Table 1. Comparison of Biological Activities of Compounds 16 and 19 with Those of Paclitaxel and Docetaxel

compound Paclitaxel Docetaxel 16 19

enhancement of tubulin polymerization (EC50, µM, ( SD) condition 1a condition 2b 8.4 ( 0.8 5.2 ( 0.7 >50 4.2 ( 0.1

1.5 ( 0.2 1.5 ( 0.2 13 ( 0.7 1.1 ( 0.1

effects on human cancer cell growth IC50 (nM) Burkitt lymphomac

prostate carcinomad

melanomad

breast carcinomad

CA46 5 3 >1000 2

PC3 4 0.9 >2500 1

LOX-IMV1 6 0.9 NTe 0.6

MCF-7 2 0.5 NT 0.2

ovarian carcinomad 1A9 4 2 NT 2

1A9PTX10 60 10 NT 20

1A9PTX22 60 20 NT 20

a Reaction mixtures (100 µL) contained 1.0 mg/mL purified bovine brain tubulin, 0.4 M monosodium glutamate (pH 6.6 in 2.5 M stock solution with HCl), 5% (v/v) dimethyl sulfoxide as drug solvent, and varying drug concentrations. Incubation was for 20 min at room temperature. Samples were centrifuged at 14000 rpm for 10 min in an Eppendorf model 5417C centrifuge at room temperature, and the protein content of 20 µL of the supernatant was determined. Without drug the supernatant contained at least 95% as much protein as the total reaction mixture. The EC50 is the drug concentration that reduced supernatant protein by 50%. The experiment was performed 3 times. b Besides tubulin and drugs, reaction mixtures contained 0.65 M monosodium glutamate and 0.1 mM GTP. Conditions were otherwise unchanged. c Cells were grown for 4 days in suspension culture. The IC50 is the drug concentration that reduced cell number by 50%. d Cells were grown for 4 days as monolayer cultures in microtiter plates. The IC50 is the drug concentration that reduced cell protein by 50%. e Not tested.

purification or on standing in CDCl3 solution. The formation of 25 presumably occurs by intramolecular attack of the carbomethoxy carbonyl group of 21 on C-5 with concomitant ring-opening, followed by a prototopic shift. A similar migration of an acetate group to C-5 with D-ring opening was observed by treating 20 with Ac2O/ DMAP in toluene at 80 °C. Since 20 could not be converted to 21, it was subjected directly to reaction with phenyllithium to afford the C-2 benzoylated analogue 22 in 43% yield. Deprotection of 22 with HF-pyridine led to compound 23 in 70% yield, and this was subjected to testing for cytotoxicity in the HCT 116 cell line. Biological Activities. Compounds 16 and 19 were examined for effects on the polymerization of purified bovine brain tubulin and on the growth of several human

cancer cell lines (Table 1). We found that 16 had negligible activity in all assays. Compound 19, however, was clearly superior in activity to paclitaxel and even docetaxel with purified tubulin, and 19 was also more inhibitory than both paclitaxel and docetaxel against the growth of most of the human cancer cell lines examined. Initial qualitative studies with tubulin were performed by turbidimetry, which demonstrated that 19 was more potent than paclitaxel, while 16 was much less active. Further work suggested that 19 would have biochemical properties similar to those of docetaxel, and no reaction condition was found where the slight activity of 16 could be demonstrated in the absence of GTP in the reaction mixture (GTP is an essential component for “normal” microtubule assembly, i.e., without drug). We had previously developed an assay for the ready quantitative

Novel Paclitaxol D-Ring Modified Analogues

comparison of paclitaxel analogues with activity superior to that of the parent drug.16 This was drug-induced assembly at room temperature in 0.4 M monosodium glutamate (pH 6.6) without GTP, with the EC50 defined as the drug concentration that reduced supernatant tubulin concentration by 50%. Although the tubulin used in current studies yields substantially lower EC50 values for paclitaxel and docetaxel (Table 1; condition 1, 8.4 and 5.2 µM), compounds such as 19, which are more active than paclitaxel, can still be readily distinguished. In this assay, weakly active compounds such as 16 are indistinguishable from those devoid of activity. Although we had previously shown that increasing the glutamate concentration allowed one to identify many such weakly active compounds,16 this was insufficient to obtain a quantitative evaluation of the partial activity of 16. We, therefore, evaluated GTP-containing reaction conditions that would permit quantitative evaluation of 16 and other poorly active paclitaxel analogues. As in the previous work, our goal was to obtain a reaction condition in which polymer formation at room temperature (measured by residual protein in supernatants following centrifugation at room temperature) would be negligible. The best results were obtained when reaction mixtures contained 0.65 M glutamate, 0.1 mM GTP, and 10 µM tubulin. As in higher glutamate alone, in this reaction condition compounds more active than paclitaxel yield EC50 values that differ little from that obtained with paclitaxel. But the partial activity of 16 could now be quantitated, and it appears to be at least 9-fold less active than paclitaxel. Despite its weak activity with tubulin, compound 16 had no effect on the growth of the first two human cancer cell lines examined (a Burkitt lymphoma and a prostate carcinoma line), and so it was not studied further. Compound 19, however, was highly active in these two lines, and we extended the analysis to a melanoma line, a breast carcinoma line, and a parental ovarian carcinoma line (1A9) and two paclitaxel-resistant lines derived from it. Paclitaxel resistance in these lines (PTX10 and PTX22) was shown to be caused by two different mutations in the dominant β-tubulin gene expressed in 1A9 cells.17 All cell lines were modestly more sensitive to docetaxel than to paclitaxel, and all cells except the prostate carcinoma line and the paclitaxel-resistant mutants were more sensitive to 19 than to docetaxel. In the current experiments the 22-fold relative resistance to paclitaxel of 1A9PTX10 and 1A9PTX22 cells as compared with the parental cells was reduced to about 4-5fold to docetaxel and about 10-12-fold to compound 19. The selenapaclitaxel analogue 23 was evaluated for cytotoxicity in the HCT 116 cell line. It showed no detectable activity, consistent with the fact that 4-acyl substitution is necessary for activity.18 Conclusion. Substitution of a sulfur for the oxygen of the oxetane ring in paclitaxel causes a significant diminution of its tubulin-assembly activity and cytotoxicity. This finding is consistent with the previous finding (16) Lin, C. M.; Jiang, Y. Q.; Chaudhary, A. G.; Rimoldi, J. M.; Kingston, D. G. I.; Hamel, E. Cancer Chemother. Pharmacol. 1996, 38, 136-140. (17) Giannakakou, P.; Sackett, D. L.; Kang, Y.-K.; Zhan, Z.; Buters, J. T. M.; Fojo, T.; Poruchynsky, M. S. J. Biol. Chem. 1997, 272, 1711817125. (18) Neidigh, K. A.; Gharpure, M. M.; Rimoldi, J. M.; Kingston, D. G. I.; Jiang, Y. Q.; Hamel, E. Tetrahedron Lett. 1994, 35, 6839-6842.

J. Org. Chem., Vol. 64, No. 8, 1999 2699

that substitution of the oxygen by a nitrogen also causes a marked reduction in activity,7 but it extends it by demonstrating that the activity loss is not due to any basicity of the heteroatom. It can thus be concluded either that this region of the pharmacophore of paclitaxel is very sensitive to steric effects or that the oxetane oxygen is acting as a hydrogen bond acceptor.

Experimental Section General Methods. All chemicals were purchased from Aldrich Chemical Co. with the exception of iodotrimethylsilane, which was obtained from Fluka. Chemicals were used without further purification, unless indicated otherwise. All anhydrous reactions were performed in oven-dried glassware under a positive pressure of argon. Anhydrous tetrahydrofuran (THF) and dimethylformamide (DMF) were purchased from Aldrich and used directly. Anhydrous CH2Cl2 was distilled from calcium hydride. All reactions were monitored by TLC (silica gel GF plates), visualized under short-wave UV light, and developed with vanillin/H2SO4 reagent spray. Analytical TLC was performed on kieselgel 60 F254 (E merck), 0.2 mm thickness. Preparative TLC was performed on silica gel GF plates (20 × 20 cm), 0.5 or 1.0 mm thickness, and purchased from Analtech. Flash chromatography was carried out on silica gel purchased from Baxter Scientific Products (S/P brand silica gel 60 Å, 230-400 mesh). Melting points were determined on a Kofler hot-stage apparatus and were uncorrected. All NMR spectra were recorded in CDCl3 using solvent (δC 77.0 ppm) or residual CHCl3 (δH 7.24 ppm) as internal standards. Coupling constants were reported in hertz. 1H and 13C NMR spectra were assigned primarily by comparison of the chemical shifts and coupling constants with those of related compounds or with the aid of 2D-NMR experiments (COSY, DEPT, HETCOR, HMQC, and HMBC). Microanalyses were obtained from Atlantic Microlabs, Inc., GA, and exact mass measurements were performed at the Nebraska Center for Mass Spectrometry. All solvent evaporation was done under reduced pressure. 2′-(tert-Butyldimethylsilyl)-7-(triethylsilyl)paclitaxel (4). To a stirred solution of 1a (1.525 g, 1.786 mmol) in anhydrous DMF (14.12 mL) were added imidazole (605 mg, 8.887 mmol) and tert-butyldimethylsilyl chloride (1.344 g, 8.917 mmol). The reaction mixture was stirred at 60 °C for 2 h and cooled to room temperature, and additional imidazole (605 mg, 8.887 mmol) and triethylsilyl chloride (0.85 mL, 5.064 mmol) were added. The reaction mixture was stirred at room temperature for 1 h and then diluted with EtOAc (200 mL). The organic phase was washed successively with water (4 × 60 mL) and brine (60 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by flash chromatography (hexanes: EtOAc, 2:1) to give 4 (1.756 g, 91%) as a white amorphous solid: mp 129-130 °C. The NMR spectra for 4 were identical with those previously reported.10 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)paclitaxel (6). To a stirred solution of 4 (510 mg, 0.47 mmol) in anhydrous CH2Cl2 (22.4 mL) at -78 °C was added benzyltrimethylammonium hydroxide (Triton B, 626 µL of 40% w/v solution in methanol). The reaction mixture was stirred at -78 °C for 10 min. The dry ice-acetone cooling bath was then replaced with a dry ice-diethylene glycol bath and the reaction mixture stirred at -10 to - 15 °C for a further 2 h. The progress of the reaction was monitored by TLC (silica gel, 50% hexanes in EtOAc), which indicated the initial formation of the compound 2′-(tert-butyldimethylsilyl)-2-debenzoyl-7-(triethylsilyl)paclitaxel10 (5) with a lower Rf than that of the starting material (4). As the reaction progressed, 5 was converted to the required product, 2′-(tert-butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7-(triethylsilyl)paclitaxel (6), which had an Rf intermediate between those of 4 and 5. After 2 h there was no starting material remaining. The major product was 6, as well as a very polar product at the origin of the TLC plate. The reaction mixture was then diluted with

2700 J. Org. Chem., Vol. 64, No. 8, 1999 cold CH2Cl2 (-78 °C, 60 mL) and quenched with cold 0.1 N HCl (0 °C, 5 mL). The CH2Cl2 layer was washed successively with with water (2 × 10 mL), 5% NaHCO3 (10 mL), H2O (5 mL), and brine (5 mL). The organic phase was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by flash chromatography (hexanes: EtOAc, 1:1) to give 6 (291 mg, 66%). The NMR data (1H and 13C) were identical with those reported previously.10 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)paclitaxel 1,2-Carbonate (7). To a solution of 6 (935 mg, 1 mmol) in anhydrous CH2Cl2 (19 mL) were added carbonyldiimidazole (3.523 g, 21.73 mmol) and imidazole (218 mg, 3.20 mmol). The reaction mixture was stirred under argon at 35-40 °C for 24 h. After this period, the reaction mixture was diluted with 200 mL of CH2Cl2 and washed successively with 5% HCl (5 × 60 mL), 5% NaHCO3 (2 × 60 mL), H2O (60 mL), and brine (60 mL). The organic phase was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by flash chromatography (40% EtOAc in hexanes) to give 7 (818 mg, 85%) as a white crystalline solid: mp 222-224 °C; 1H NMR δ -0.28 (s, 3H), -0.11 (s, 3H), 0.59 (q, 6H, J ) 7.83 Hz), 0.82 (s, 9H), 0.92 (t, 9H, J ) 7.86 Hz), 1.16 (s, 3H), 1.18 (s, 3H), 1.65 (s, 3H), 1.98 (m, 1H), 2.16 (s, 6H), 2.47 (dd, 1H, J ) 9.77, 15.26 Hz), 2.57 (m, 1H), 2.75 (dd, 1H, J ) 4.58, 15.71 Hz), 2.99 (d, 1H, J ) 6.11 Hz), 4.21 (dd, 1H, J ) 6.87, 10.53 Hz), 4.39 (d, 1H, J ) 5.34 Hz), 4.53 (d, 1H, J ) 8.70 Hz), 4.53 (d, 1H, J ) 1.68 Hz), 4.59 (d, 1H, J ) 8.69 Hz), 4.94 (dd, 1H, J ) 2.29, 9.31 Hz), 5.15 (s, 1H), 5.85 (dd, 1H, J ) 1.6, 9.0 Hz), 5.89 (brt, 1H, J ) 7.0 Hz), 6.43 (s, 1H), 7.26-7.54 (m, 8H), 7.75 (d, 2H, J ) 8.32 Hz); 13C NMR δ -5.88, -5.58, 5.21, 6.75, 9.92, 16.54, 18.23, 19.22, 20.81, 25.50, 26.40, 32.46, 37.92, 41.05, 48.61, 55.24, 60.07, 71.04, 72.17, 73.19, 75.14, 76.00, 80.23, 81.17, 87.51, 89.80, 126.68, 127.05, 127.97, 128.55, 128.86, 132.14, 133.41, 133.98, 139.13, 141.29, 152.69, 167.38, 169.18, 170.42, 202.32. FABMS m/z 962 [M + H]+. Anal. Calcd for C51H71NO13Si2: C, 63.65; H, 7.44; N, 1.46. Found: C, 63.77; H, 7.61; N, 1.46. 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)-5-r-iodo-20-hydroxy-D-seco-paclitaxel 1,2Carbonate (8). To a stirred solution of 7 (389 mg, 0.40 mmol) in anhydrous CH2Cl2 (23 mL) at -78 °C was added iodotrimethylsilane (220 µL, 1.62 mmol). The reaction mixture was stirred at -78 °C for 45 min and quenched with CH2Cl2 (180 mL) precooled to -78 °C, followed by water (100 mL, 0 °C). The organic layer was separated and washed successively with 0.05 M Na2S2O4 (2 ×100 mL), water (25 mL), and brine (25 mL). The CH2Cl2 layer was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by flash silica gel chromatography (EtOAc:hexanes, 2:3) to give 413 mg (93%) of 8 as a white crystalline solid: mp 153-155 °C; 1H NMR δ -0.36 (s, 3H), -0.15 (s, 3H), 0.56 (q, 6H, J ) 7.93 Hz), 0.75 (s, 9H), 0.92 (t, 9H, J ) 7.94 Hz), 1.08 (s, 3H), 1.11 (s, 3H), 1.18 (s, 3H), 2.03 (m, 1H), 2.15 (s, 3H), 2.23 (td, 1H, J ) 3.21, 15.72 Hz), 2.36 (dd, 1H, J ) 9.77, 15.56 Hz), 2.45 (d, 3H, J ) 1.07 Hz), 3.16 (dd, 1H, J ) 5.72, 15.49 Hz), 3.54 (d, 1H, J ) 10.98 Hz), 3.59 (d, 1H, J ) 4.88 Hz), 3.99 (d, 1H, J ) 11.14 Hz), 4.26 (d, 1H, J ) 4.88 Hz), 4.50 (dd, 1H, J ) 3.74, 10.61 Hz), 4.59 (d, 1H, J ) 1.83 Hz), 5.07 (t, 1H, J ) 2.98 Hz), 5.83 (dd, 1H, J ) 1.30, 8.17 Hz), 6.11 (dd, 1H, J ) 8.17 Hz), 6.54 (s, 1H), 7.17 (d, 1H, J ) 8.24 Hz), 7.26-7.54 (m, 8H), 7.74 (d, 2H, J ) 7.02 Hz); 13C NMR δ -6.00, -5.61, 5.14, 6.75, 13.38, 17.67, 18.16, 19.70, 20.79, 25.45, 26.34, 32.49, 38.90, 41.01, 45.34, 46.52, 55.75, 60.73, 62.01, 70.61, 71.02, 73.58, 75.04, 75.43, 82.13, 89.74, 126.92, 127.06, 127.61, 128.35, 128.90, 132.01, 133.01, 133.79, 138.54, 143.22, 152.71, 167.32, 169.08, 171.18, 201.42; HRFABMS calcd for C51H72NO13Si2INa [M+Na]+ m/z 1112.3485, found 1112.3488. Anal. Calcd for C51H72NO13Si2I: C, 56.19; H, 6.66; N, 1.28. Found: C, 55.90; H, 6.62; N, 1.52. 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)-5-r-iodo-4,20-epoxy-D-seco-paclitaxel 1,2Carbonate (10). To a stirred solution of 8 (288 mg, 0.26 mmol) and DMAP (425 mg, 3.48 mmol) in anhydrous CH2Cl2 at 0 °C was added trifluoromethanesulfonyl chloride (300 µL, 2.82 mmol). The reaction mixture was stirred at 0 °C for 2 min, at

Gunatilaka et al. which time the solution became very viscous. It was kept in the 0 °C cooling bath for a total of 8 min and then left at room temperature for 50 min. After this period, 100 mL of EtOAc and 15 mL water were added to the reaction flask. The organic layer was separated from the aqueous layer, washed successively with water (2 × 15 mL) and brine (15 mL), and dried over anhydrous Na2SO4. The organic phase was evaporated to dryness and the crude product purified by flash chromatography (EtOAc:hexanes, 3:7) to give 10 (266 mg, 96%) as a colorless crystalline solid: mp 241-243 °C; 1H NMR δ -0.32 (s, 3H), -0.13 (s, 3H), 0.56 (q, 6H, J ) 7.88 Hz), 0.80 (s, 9H), 0.89 (t, 9H, J ) 7.94 Hz), 1.16 (s, 3H), 1.25 (s, 3H), 1.31 (s, 3H), 2.16 (s, 3H), 2.28 (m, 1H), 2.41 (s, 3H), 2.39 (m, 1H), 2.57 (dd, 1H, J ) 9.77, 15.87 Hz), 2.68 (dd, 1H, J ) 6.87, 15.87 Hz), 2.94 (d, 1H, J ) 4.88 Hz), 3.86 (d, 1H, J ) 4.73 Hz), 3.87 (d, 1H, J ) 4.43 Hz), 4.18 (d, 1H, J ) 5.03 Hz), 4.21 (m, 1H), 4.50 (d, 1H, J ) 1.99 Hz), 4.53 (m, 1H), 5.71 (dd, 1H, J ) 1.53, 8.85 Hz), 6.24 (t, 1H, J ) 7.79 Hz), 6.54 (s, 1H), 7.07 (d, 1H, J ) 8.69 Hz), 7.26-7.53 (m, 8H), 7.80 (d, 2H, J ) 7.02 Hz); 13C NMR δ -5.65, -5.32, 5.12, 6.63, 12.23, 17.34, 18.17, 19.86, 20.75, 25.56, 26.30, 33.18, 39.36, 40.10, 41.29, 41.89, 55.85, 58.79, 60.24, 62.82, 69.95, 70.56, 75.67, 75.83, 80.72, 89.08, 126.85, 127.05, 127.73, 128.52, 128.66, 131.58, 131.87, 134.33, 138.57, 143.96, 152.28, 166.55, 169.08, 171.16, 201.54; HRFABMS calcd for C51H70INO12Si2Na [M + Na]+ m/z 1094.3379, found 1094.3347. Anal. Calcd for C51H70INO12Si2: C, 57.13; H, 6.68; N, 1.31. Found: C, C, 57.04; H, 6.50; N, 1.27. 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)-5,20-deoxy-5,20-sulfinylpaclitaxel 1,2-Carbonate (11) and 2′-(tert-Butyldimethylsilyl)-2-debenzoyl4-deacetyl-7-(triethylsilyl)-5,20-deoxy-5,20-dithiapaclitaxel 1,2-Carbonate (12). To a mixture of 10 (94.3 mg, 0.088 mmol) and Li2S (25 mg, 0.544 mmol) was added anhydrous THF (7.8 mL). The resulting suspension was stirred under argon at room temperature for 28 h (TLC control for the disappearance of 10). The reaction mixture was diluted with EtOAc (115 mL) and 0.05 M Na2S2O4 (25 mL). The organic layer was separated from the aqueous phase and washed successively with 0.05 M Na2S2O4 solution (25 mL), water (15 mL), and brine (15 mL). The organic phase was dried over anhydrous Na2SO4 and evaporated to dryness. The major product had a lower Rf than 10, and 13C NMR indicated that the 1, 2-carbonate protecting group was absent in this product. Hence, the crude product was treated with carbonyldiimidazole in order to synthesize the 1,2-carbonate-protected compound; i.e to the previously dried crude product in anhydrous CH2Cl2 (1.9 mL) were added carbonyldiimidazole (201 mg, 1.24 mmol) and imidazole (56 mg, 0.82 mmol). The reaction mixture was stirred under argon at room temperature for 12 h and worked up as described in the preparation of 7. The crude product was purified by preparative TLC (15% EtOAc in hexanes) to give two products. The major product was 2′-(tert-butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7-(triethylsilyl)-5,20-deoxy-5,20sulfinylpaclitaxel 1,2-carbonate (11) (48.1 mg, 56%) and the minor product was 2′-(tert-butyldimethylsilyl)-2-debenzoyl-4deacetyl-7-(triethylsilyl)-5,20-deoxy-5,20-dithiapaclitaxel 1,2carbonate (12) (11.2 mg, 13%). Compounds 11 and 12 had very similar Rf values and required multiple developments by preparative TLC to separate them. With a large excess of Li2S (103.5 mg, 2.25 mmol) and 10 (64.4 mg, 0.06 mmol) in 5.4 mL of THF over 28 h at room temperature, the major product was 12 (25.4 mg, 42%) and the minor product was 11 (11.6 mg, 20%). Compound 11: 1H NMR δ -0.29 (s, 3H), -0.11 (s, 3H), 0.58 (q, 6H, J ) 7.78 Hz), 0.83 (s, 9H), 0.91 (t, 9H, J ) 7.94 Hz), 1.157 (s, 3H), 1.164 (s, 3H), 1.63 (s, 3H), 2.155 (s, 3H), 2.165 (s, 3H), 2.34 (m, 1H), 2.51 (m, 2H), 2.75 (d, 1H, J ) 5.34 Hz), 2.90 (dd, 1H, J ) 4.66, 15.34 Hz), 3.10 (d, 1H, J ) 11.75 Hz), 3.47 (t, 1H, J ) 9.00 Hz), 3.78 (d, 1H, J ) 11.90 Hz), 4.08 (m, 1H), 4.29 (d, 1H, J ) 5.34 Hz), 4.52 (d, 1H, J ) 1.53 Hz), 5.35 (bs, 1H), 5.87 (m, 1H), 5.93 (d, 1H, J ) 8.85 Hz), 6.41 (s, 1H), 7.27-7.52 (m, 8H), 7.78 (d, 2H, J ) 7.02 Hz); 13C NMR δ -5.89, -5.56, 5.25, 6.76, 11.32, 16.59, 18.23, 19.20, 20.80, 25.52, 26.46, 32.48, 39.73, 40.84, 41.08, 47.79, 51.44, 55.29, 60.28, 71.14, 72.44, 75.30, 75.95, 78.93, 81.44, 89.84, 126.63, 127.07, 127.92, 128.59, 128.83, 132.07, 133.58, 133.69, 139.33,

Novel Paclitaxol D-Ring Modified Analogues 141.35, 152.78, 167.31, 169.21, 170.40, 202.27; HRFABMS calcd for C51H72NO12Si2S [M + H]+ m/z 978.4314, found 978.4290 (2.4 ppm dev). Compound 12: 1H NMR δ -0.28 (s, 3H), -0.12 (s, 3H), 0.59 (q, 6H, J ) 7.78 Hz), 0.83 (s 9H), 0.91 (t, 9H, J ) 7.94 Hz), 1.18 (s, 6H), 1.28 (s, 3H), 2.16 (s, 3H), 2.18 (d, 3H, J ) 1.22 Hz), 2.22 (m, 1H), 2.47 (m, 2H), 3.04 (dd, 1H, J ) 4.43, 15.72 Hz), 3.32 (bs, 2H), 3.37 (d, 1H, J ) 5.34 Hz), 3.38 (m, 1H), 4.20 (dd, 1H, J ) 4.50, 11.37 Hz), 4.29 (d, 1H, J ) 5.04 Hz), 4.49 (d, 1H, J ) 1.53 Hz), 4.66 (bs, 1H), 5.78 (dd, 1H, J ) 1.30, 8.47 Hz), 5.92 (dd, 1H, J ) 3.28, 10.00 Hz), 6.53 (s, 1H), 7.22 (d, 1H, J ) 8.54 Hz), 7.27-7.53 (m, 8H), 7.76 (d, 2H, J ) 6.87 Hz); 13C NMR δ -5.86, -5.58, 5.19, 6.76, 10.88, 16.73, 18.23, 19.11, 20.80, 25.52, 26.68, 32.35, 35.48, 41.31, 49.32, 49.43, 55.31, 61.69, 64.95, 71.17, 72.57, 75.16, 75.64, 81.32, 86.13, 89.71, 126.72, 127.02, 127.94, 128.55, 128.78, 131.93, 133.70, 133.76, 139.11, 141.30, 152.37, 167.10, 169.19, 170.40, 201.86; HRFABMS calcd for C51H72NO12Si2S2 [M + H]+ m/z 1010.4035, found 1010.4051 (1.6 ppm dev). 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)-5,20-deoxy-5,20-sulfonylpaclitaxel 1,2-Carbonate (13). To a stirred solution of 11 (12.7 mg, 0.013 mmol) in anhydrous CH2Cl2 (250 µL) at 0 °C was added dropwise over 5 min a solution of 3-chloroperbenzoic acid (12.7 mg, 0.074 mmol) in 250 µL of anhydrous CH2Cl2. The reaction mixture was stirred at 0 °C for 30 min. The cooling bath was removed and the reaction mixture stirred at room temperature for 50 min. The reaction mixture was diluted with CH2Cl2 (45 mL) and saturated sodium sulfite (15 mL). The organic layer was separated from the aqueous phase and washed successively with 5% NaHCO3 (2 × 17 mL), water (15 mL), and brine (15 mL). The organic phase was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by preparative TLC (EtOAc:hexanes, 3:7) to give 13 (8 mg, 61%): 1H NMR δ -0.21 (s, 3H), -0.08 (s, 3H), 0.62 (q, 6H, J ) 7.71 Hz), 0.87 (s, 9H), 0.94 (t, 9H, J ) 7.86 Hz), 1.19 (s, 3H), 1.20 (s, 3H), 1.38 (s, 3H), 2.18 (s, 3H), 2.20 (d, 3H, J ) 1.07 Hz), 2.43 (m, 2H), 2.50 (dd, 1H, J ) 9.77, 15.72 Hz), 2.70 (dd, 1H, J ) 4.58, 15.72 Hz), 3.34 (d, 1H, J ) 5.49 Hz), 4.22 (t, 1H, J ) 8.39 Hz), 4.28 (dd, 1H, J ) 4.28, 15.26 Hz), 4.33 (d, 1H, J ) 5.49 Hz), 4.48 (m, 1H), 4.58 (d, 1H, J ) 1.53 Hz), 5.89 (d, 1H, J ) 9.62 Hz), 5.94 (dd, 1H, J ) 3.37, 9.77 Hz), 6.24 (br s, 1H), 6.46 (s,1H), 7.28-7.56 (m, 8H), 7.77 (d, 2H, J ) 7.02 Hz); 13C NMR δ -5.73, -5.54, 5.18, 6.73, 10.78, 16.50, 18.26, 19.16, 20.76, 25.55, 26.37, 28.35, 32.52, 41.14, 50.43, 54.71, 59.74, 63.00, 70.70, 71.18, 75.23, 75.68, 77.65, 80.84, 82.82, 89.98, 127.02, 127.17, 128.17, 128.64, 128.85, 132.26, 133.11, 133.45, 138.66, 141.58, 152.03, 167.15, 169.13, 170.47, 200.95; HRFABMS calcd for C51H72NO14Si2S [M + H]+ m/z 1010.4212, found 1010.4206 (0.6 ppm dev). 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-4methoxycarbonyl-7-(triethylsilyl)-5,20-deoxy-5,20-sulfinylpaclitaxel 1,2-Carbonate (14). To a stirred solution of 11 (23.3 mg, 0.024 mmol) in anhydrous THF (730 µL), at -78 °C under argon, was added 90 µL (0.09 mmol) of a 1.0 M of solution of lithium bis(trimethysilyl)amide (LHMDS) in THF. The reaction mixture was stirred at -78 °C for 7 min. The reaction flask was removed from the -78 °C bath and stirred at room temperature for 1 min. It was then placed in the -78 °C bath and stirred for 2 min, and freshly distilled methyl chloroformate (20 µL, 0.26 mmol) was added. The reaction mixture was stirred at -78 °C for 10 min and quenched with EtOAc (60 mL). The EtOAc layer was washed successively with 5% HCl (2 × 20 mL), 5% NaHCO3 (2 × 20 mL), water (15 mL), and brine (20 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by preparative TLC (hexanes:EtOAc, 4:1) to give unreacted 11 (9.5 mg, 41%) and 14 (9.7 mg, 39%): 1H NMR δ -0.39 (s, 3H), -0.07 (s, 3H), 0.56 (q, 6H, J ) 7.73 Hz), 0.77 (s, 9H), 0.88 (t, 9H, J ) 7.94 Hz), 1.20 (s, 3H), 1.27 (s, 3H), 1.87 (s, 3H), 2.07 (d, 3H, J ) 1.22 Hz), 2.14 (s, 3H), 2.17 (m, 2H), 2.47 (dd, 1H, J ) 9.69, 15.64 Hz), 2.64 (m, 1H), 3.47 (d, 1H, J ) 6.11 Hz), 3.54 (d, 1H, J ) 12.82 Hz), 3.60 (d, 1H, J ) 12.97 Hz), 3.99 (s, 3H), 4.17 (dd, 1H, J ) 5.19, 10.99 Hz), 4.31 (dd, 1H, J ) 6.18, 10.45 Hz), 4.44 (d, 1H, J ) 6.10 Hz), 4.71 (d, 1H, J ) 1.83 Hz), 5.86 (d, 1H, J ) 8.55 Hz), 6.22 (t,

J. Org. Chem., Vol. 64, No. 8, 1999 2701 1H, J ) 8.02 Hz), 6.41 (s, 1H), 7.05 (d, 1H, J ) 9.31 Hz), 7.277.53 (m, 8H), 7.78 (d, 2H, J ) 7.02 Hz); 13C NMR δ -5.96, -5.28, 5.24, 6.73, 11.99, 15.34, 18.06, 20.39, 20.77, 25.48, 25.59, 32.24, 35.37, 40.35, 41.24, 41.67, 46.41, 54.88, 56.98, 59.90, 68.90, 72.10, 75.16, 75.78, 81.31, 89.31, 89.93, 126.35, 127.02, 127.62, 128.51, 128.79, 130.87, 131.78, 134.18, 138.62, 143.75, 150.86, 152.37, 166.73, 169.08, 171.47, 201.89; HRFABMS calcd for C53H73NO14Si2SLi [M + Li]+ m/z 1042.4450, found 1042.4465 (-1.4 ppm dev). 2′-(tert-Butyldimethylsilyl)-4-deacetyl-4-methoxycarbonyl-7-(triethylsilyl)-5,20-deoxy-5,20-sulfinylpaclitaxel (15). A solution of 14 (9.0 mg, 0.009 mmol) in anhydrous THF (160 µL) was stirred at -78 °C under argon. To the solution of 14 was added phenyllithium (0.045 mmol, 25 µL of a 1.8 M solution in cyclohexanes-ether). The reaction mixture was stirred at -78 °C for 3 min and quenched with EtOAc (25 mL). The EtOAc layer was washed successively with 5% HCl (2 × 8 mL), 5% NaHCO3 (2 × 6 mL), water (6 mL), and brine (10 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness. Purification of the crude product by preparative TLC (hexanes:EtOAc, 4:1) gave 5.9 mg (61%) of 15: 1H NMR δ -0.37 (s, 3H), -0.07 (s, 3H), 0.56 (q, 6H, J ) 7.68 Hz), 0.75 (s, 9H), 0.90 (t, 9H, J ) 7.94 Hz), 1.11 (s, 3H), 1.19 (s, 3H), 1.79 (s, 3H), 2.02-2.21 (m, 2H), 2.11 (s, 3H), 2.15 (s, 3H), 2.38 (dd, 1H, J ) 8.63, 15.65 Hz), 2.54 (m, 1H), 3.13 (d, 1H, J ) 12.06 Hz), 3.48 (d, 1H, J ) 12.20 Hz), 3.96 (d, 1H, J ) 7.17 Hz), 4.05 (dd, 1H, J ) 6.94, 10.61 Hz), 4.09 (s, 3H), 4.33 (dd, 1H, J ) 5.65, 11.30 Hz), 4.73 (d, 1H, J ) 1.83 Hz), 5.66 (d, 1H, J ) 7.33 Hz), 5.93 (d, 1H, J ) 8.32 Hz), 6.26 (t, 1H, J ) 8.07 Hz), 6.44 (s, 1H), 7.09 (d, 1H, J ) 9.30 Hz), 7.26-7.57 (m, 11H), 7.76 (d, 2H, J ) 7.18 Hz), 8.12 (d, 2H, J ) 7.02 Hz); 13C NMR δ -6.06, -5.29, 5.31, 6.77, 11.85, 14.62, 18.11, 20.90, 21.06, 25.47, 26.61, 35.95, 40.03, 42.80, 42.91, 48.90, 55.13, 56.91, 58.67, 70.15, 72.36, 75.19, 75.29, 75.32, 78.88, 90.74, 126.37, 126.97, 127.58, 128.50, 128.77, 128.83, 129.59, 130.20, 131.69, 132.70, 133.64, 134.35, 138.79, 140.83, 151.42, 166.74, 167.30, 169.32, 171.36, 201.63; HRFABMS calcd for C59H79NO14Si2SLi [M + Li]+ m/z 1120.4920, found 1120.4948 (-2.5 ppm dev). 4-Deacetyl-4-methoxycarbonyl-5,20-deoxy-5,20-sulfinylpaclitaxel (16). Compound 15 (7.3 mg, 0.007 mmol) was dissolved in THF (830 µL) in a 5 mL Teflon flask. The solution of 15 was stirred at 0 °C and a cold solution (0 °C) of hydrogen fluoride-pyridine (60 µL) added. The reaction mixture was stirred at room temperature for 9 h and diluted with EtOAc (25 mL). The organic layer was washed successively with 5% NaHCO3 (2 × 6 mL), 5% HCl (2 × 6 mL), water (6 mL), and brine (10 mL). The organic phase was dried over anhydrous Na2SO4 and evaporated to dryness. The crude product was purified by preparative TLC (hexanes:EtOAc, 1:1) to give 4.4 mg (76%) of 16: 1H NMR δ 1.11 (s, 3H), 1.19 (s, 3H), 1.75 (s, 3H), 1.86 (d, 3H, J ) 1.07 Hz), 2.19 (m, 1H), 2.22 (s, 3H), 2.33 (m, 2H), 2.56 (m, 2H), 3.12 (d, 1H, J ) 11.90 Hz), 3.49 (d, 1H, J ) 12.20 Hz), 3.58 (d, 1H, J ) 3.66 Hz), 3.90 (d, 1H, J ) 7.17 Hz), 3.93 (s, 3H), 4.01 (dd, 1H, J ) 7.17, 10.38 Hz), 4.22 (m, 1H), 4.83 (dd, 1H, J ) 2.29, 3.66 Hz), 5.66 (d, 1H, J ) 7.17 Hz), 5.94 (d, 1H, J ) 9.00 Hz), 6.16 (t, 1H, J ) 8.47 Hz), 6.24 (s, 1H), 7.03 (d, 1H, J ) 9.15 Hz), 7.31-7.59 (m, 11H), 7.76 (d, 2H, J ) 7.02 Hz), 8.12 (d, 2H, J ) 7.17 Hz); 13C NMR δ 11.25, 15.18, 20.87, 21.56, 27.02, 36.01, 36.17, 38.43, 42.77, 43.00, 47.60, 54.33, 56.37, 58.87, 71.87, 72.14, 73.37, 75.25, 75.67, 79.06, 90.87, 126.88, 127.01, 128.07, 128.71, 128.82, 128.86, 129.44, 130.18, 131.88, 132.39, 133.74, 133.83, 138.49, 142.53, 151.99, 166.64, 167.14, 171.38, 172.70, 203.46; HRFABMS calcd for C47H51NO14SNa [M + Na]+ m/z 908.2928, found 908.2933 (-0.8 ppm dev). 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-4methoxycarbonyl-7-(triethylsilyl)paclitaxel 1,2-Carbonate (17). Acetylation of 7 to give 17 was done using the same methodology as for the preparation of 14 from 11. Reaction of 7 (28.4 mg, 0.03 mmol) in anhydrous THF (906 µL) with LHMDS (96 µL) and methyl chloroformate (24 µL) gave 20.8 mg (69%) of 17 after workup and preparative TLC (hexanes: EtOAc), 4:1): 1H NMR δ -0.38 (s, 3H), -0.08 (s, 3H), 0.56 (q, 6H, J ) 7.58 Hz), 0.77 (s, 9H), 0.89 (t, 9H, J ) 7.94 Hz), 1.21

2702 J. Org. Chem., Vol. 64, No. 8, 1999 (s, 3H), 1.28 (s, 3H), 1.75 (s, 3H), 1.94 (dd, 1H, J ) 10.76, 14.73 Hz), 2.04 (d, 3H, J ) 0.61 Hz), 2.14 (s, 3H), 2.24 (dd, 1H, J ) 7.71, 15.80 Hz), 2.52 (dd, 1H, J ) 9.61, 15.72 Hz), 2.62 (m, 1H), 3.53 (d, 1H, J ) 5.65 Hz), 3.96 (s, 3H), 4.39 (dd, 1H, J ) 7.1, 9.54 Hz), 4.51 (m, 2H), 4.69 (d, 1H, J ) 1.53 Hz), 4.72 (d, 1H, J ) 9.31 Hz), 5.09 (d, 1H, J ) 8.69 Hz), 5.77 (d, 1H, J ) 9.46 Hz), 6.22 (t, 1H, J ) 8.17 Hz), 6.43 (s, 1H), 7.01 (d, 1H, J ) 9.31 Hz), 7.26-7.54 (m, 8H), 7.77 (d, 2H, J ) 6.86 Hz); 13C NMR δ -5.91, -5.26, 5.16, 6.71, 10.06, 15.19, 18.06, 20.47, 20.73, 25.46, 32.09, 37.86, 41.40, 43.54, 54.96, 57.05, 59.92, 69.24, 71.49, 75.01, 75.74, 75.79, 80.98, 81.86, 83.53, 89.65, 126.36, 127.00, 127.72, 128.57, 128.78, 131.53, 131.79, 134.13, 138.41, 143.68, 151.57, 152.38, 166.82, 169.03, 171.55, 201.72; HRFABMS calcd for C53H74NO15Si2 [M + H]+ m/z 1020.4597, found 1020.4568 (2.8 ppm dev). 2′-(tert-Butyldimethylsilyl)-4-deacetyl-4-methoxycarbonyl-7-(triethylsilyl)paclitaxel (18). The benzoylation of 17 to give 18 was done using the same methodology as for the preparation of 15 from 14. Benzoylation of 17 (13.5 mg, 0.013 mmol) in anhydrous THF (250 µL) with phenyllithium (38 µL) afforded after workup and preparative TLC (hexanes:EtOAc, 3:1) 10.2 mg (70%) of 18: 1H NMR δ -0.35 (s, 3H), -0.05 (s, 3H), 0.56 (q, 6H, J ) 7.58 Hz), 0.77 (s, 9H), 0.91 (t, 9H, J ) 7.93 Hz), 1.13 (s, 3H), 1.20 (s, 3H), 1.69 (s, 3H), 1.91 (m, 1H), 2.06 (s, 3H), 2.07-2.15 (m, 1H), 2.16 (s, 3H), 2.41 (dd, 1H, J ) 8.70, 15.72 Hz), 2.53 (m, 1H), 3.95 (d, 1H, J ) 7.48 Hz), 4.05 (s, 3H), 4.24 (d, 1H, J ) 8.70 Hz), 4.34 (d, 1H, J ) 8.39 Hz), 4.45 (dd, 1H, J ) 6.56, 10.68 Hz), 4.69 (d, 1H, J ) 1.68 Hz), 4.98 (d, 1H, J ) 7.94 Hz), 5.70 (d, 1H, J ) 6.87 Hz), 5.79 (d, 1H, J ) 8.39 Hz), 6.25 (t, 1H, J ) 8.78 Hz), 6.45 (s, 1H), 7.08 (d, 1H, J ) 9.01 Hz), 7.27-7.58 (m, 11H), 7.74 (d, 2H, J ) 7.18 Hz), 8.11 (d, 2H, J ) 7.17 Hz); 13C NMR δ -5.94, -5.23, 5.24, 6.74, 10.12, 14.47, 18.11, 20.87, 21.04, 25.47, 26.51, 35.60, 37.16, 43.20, 46.72, 55.36, 56.66, 58.30, 70.64, 72.04, 74.86, 75.07, 75.15, 76.10, 78.50, 83.29, 84.01, 126.41, 126.96, 127.73, 128.58, 128.73, 128.76, 129.22, 130.13, 131.73, 133.67, 133.69, 134.22, 138.68, 140.41, 152.33, 166.85, 167.07, 169.30, 171.42, 201.57; HRFABMS calcd for C59H80NO15Si2 [M + H]+ m/z 1098.5067, found 1098.5101 (-3.2 ppm dev). 4-Deacetyl-4-methoxycarbonylpaclitaxel (19). Deprotection of the C-2′ and C-7 position in 18 to give 19 was done using the same methodology as that used to convert 15 to 16. Deprotection of the C-7 and C-2′ positions in 18 (13.1 mg, 0.012 mmol) with HF-pyridine (100 µL) in anhydrous THF (1.5 mL) afforded after preparative TLC (hexanes:EtOAc, 1:1) compound 19 (9 mg, 87%) as an amorphous solid: 1H NMR δ 1.12 (s, 3H), 1.21 (s, 3H), 1.66 (s, 3H), 1.82 (s, 3H), 1.87 (m, 1H), 2.22 (s, 3H), 2.30 (dd, 1H, J ) 9.09, 15.49 Hz), 2.43 (dd, 1H, J ) 8.55, 15.72 Hz), 2.52 (m, 1H), 3.62 (bs, 1H), 3.80 (s, 3H), 3.84 (d, 1H, J ) 7.02 Hz), 4.20 (d, 1H, J ) 8.70 Hz), 4.33 (d, 1H, J ) 8.54 Hz), 4.36 (dd, 1H, J ) 6.72, 10.84 Hz), 4.79 (d, 1H, J ) 1.83 Hz), 4.96 (dd, 1H, J ) 1.83, 9.46 Hz), 5.69 (d, 1H, J ) 7.02 Hz), 5.82 (dd, 1H, J ) 1.99, 9.01 Hz), 6.18 (t, 1H, J ) 8.32 Hz), 6.25 (s, 1H), 6.94 (d, 1H, J ) 9.15 Hz), 7.317.60 (m, 11H), 7.72 (d, 2H, J ) 7.01 Hz), 8.14 (d, 2H, J ) 7.18 Hz); 13C NMR δ 9.63, 14.96, 20.87, 21.72, 26.87, 35.38, 35.82, 43.08, 45.87, 54.74, 56.00, 58.33, 71.99, 72.30, 73.01, 74.87, 75.59, 75.99, 78.84, 83.14, 84.09, 127.01, 127.10, 128.28, 128.69, 128.74, 128.96, 129.11, 130.23, 131.91, 133.37, 133.73, 138.42, 142.08, 153.12, 166.89, 167.04, 171.32, 172.97, 203.48; HRFABMS calcd for C47H51NO15Li [M + Li]+ m/z 876.3419, found 876.3402 (1.9 ppm dev). 2′-(tert-Butyldimethylsilyl)-2-debenzoyl-4-deacetyl-7(triethylsilyl)-5,20-deoxy-5,20-selenapaclitaxel 1,2-Carbonate (20). To a stirred solution of Superhydride (0.6 mL, 1 M solution in THF, 0.6 mmol) was added selenium powder (25.3 mg, 0.32 mmol) under argon at room temperature. A further 3.7 mg (0.05 mmol) of selenium powder was added and the white suspension turned pale red. Sufficient Superhydride was syringed into the solution until the latter changed to a light pink color. The resulting suspension was stirred at room temperature for 20 min and cooled to -20 °C, and after 5 min a solution of epoxide 10 (7.0 mg, 0.0065 mmol) in anhydrous THF (0.25 mL) was added. The cooling bath was removed and stirring continued at room temperature for 2 h. The reaction

Gunatilaka et al. mixture was worked up by adding excess EtOAc and a few crystals of BHT (2,6-di-tert-butyl-4-methylphenol). The resulting solution was filtered twice through a short column of silica gel and the filtrate evaporated to dryness. The crude product was purified by preparative TLC (30% hexanes in EtOAc) to give the selenapaclitaxel analogue 20 as a white foam (4.5 mg, 67%): 1H NMR δ -0.28 (s, 3H), -0.10 (s, 3H), 0.60 (q, 6H, J ) 7.9 Hz), 0.80 (s, 9H), 0.82 (t, 9H, J ) 7.9 Hz), 1.18 (br s, 6H), 1.60 (br s, 3H), 2.16 (br s, 6H), 2.50 (m, 1H), 2.55 (m, 1H), 2.65 (d, 1H, J ) 5.0 Hz), 3.00 (d, 1H, J ) 11.0 Hz), 3.03 (m, 1H), 3.35 (t, 1H, J ) 9.2 Hz), 3.94 (d, 1H, J ) 11.0 Hz), 4.01 (m, 1H), 4.27 (d, 1H, J ) 5.0 Hz), 4.51 (d, 1H, J ) 1.7 Hz), 5.38 (s, 1H), 5.86 (m, 1H), 5.92 (dd, 1H, J ) 1.7, 8.9 Hz), 6.41 (s, 1H), 7.20-7.60 (m, 8H), 7.80 (d, 2H, J ) 8.0 Hz); 13C NMR δ -5.8, -5.5, 5.3, 6.7, 11.6, 16.6, 18.3, 19.8, 20.8, 25.5, 26.5, 28.4, 32.4, 37.5, 41.2, 41.6, 52.5, 55.4, 60.6, 71.2, 72.7, 75.3, 75.9, 80.9, 81.5, 89.8, 126.7, 127.1, 127.9, 128.6, 128.8, 132.0, 133.7, 139.4, 141.4, 152.9, 167.3, 169.2, 170.4, 202.2; HRFABMS calcd for C51H71NO12Si2SeNa [M + Na]+ m/z 1048.3578, found 1048.3587. Acylation of Selenapaclitaxel 20. To a stirred solution of 20 (7.0 mg, 0.007 mmol) in anhydrous THF at -78 °C under argon was added 20 µL (0.02 mmol, 3 equiv) of a 1.0 M of solution of LHMDS in THF. The reaction mixture was stirred at -78 °C for 5 min. The reaction flask was removed from the -78 °C bath and stirred at room temperature for 1 min. At the end of this period, the reaction flask was returned to the -78 °C bath and stirred for 2 min, and freshly distilled methyl chloroformate (5 µL, 0.06 mmol, 9 equiv) was added. The reaction mixture was stirred at -78 °C for 5 min and quenched with EtOAc (60 mL). The EtOAc layer was washed successively with 5% HCl (2 × 20 mL), 5% NaHCO3 (2 × 20 mL), water (15 mL), and brine (20 mL). The organic layer was dried over anhydrous Na2SO4 and evaporated to dryness, and the residue was purified by preparative TLC (hexanes:EtOAc, 4:1) to yield a mixture of unreacted starting material (20, 1.0 mg, 14%) and 25 (3.5 mg, 48%): 1H NMR δ -0.24 (s, 3H), -0.01 (s, 3H), 0.51 (q, 6H, J ) 7.9 Hz), 0.86 (s, 9H), 0.87 (t, 9H, J ) 7.9 Hz), 1.19 (s, 3H), 1.22 (s, 3H), 1.26 (s, 3H), 1.80 (m, 1H), 2.17 (s, 3H), 2.22 (s, 3H), 2.32 (m, 2H), 2.51 (m, 1H), 3.62 (s, 3H), 3.78 (d, 1H, J ) 5.2 Hz), 4.35 (m, 1H), 4.68 (d, 1H, J ) 1.8 Hz), 5.24 (br t, 1H), 5.48 (br s, 1H), 5.57 (dd, 1H, J ) 1.7, 8.6 Hz), 5.92 (br s, 1H), 6.23 (m, 1H), 6.57 (s, 1H), 7.08 (d, 1H, J ) 8.7 Hz), 7.22-7.54 (m, 8H), 7.76 (d, 2H, J ) 7.0 Hz); 13C NMR δ -5.7, -5.2, 6.6, 10.4, 15.6, 18.2, 19.8, 20.8, 25.5, 26.2, 33.1, 35.8, 41.2, 43.6, 55.1, 55.7, 62.3, 69.5, 69.6, 74.7, 75.5, 81.8, 82.4, 89.4, 121.7, 126.8, 126.9, 127.8, 128.4, 128.7, 131.7, 136.6, 139.1, 143.0, 154.3, 166.3, 169.3, 171.3, 202.7; FABMS calcd for C53H75NO14Si2Se [M + H]+ m/z 1083, found m/z (rel int) 1083 [M + H]+ (7%), 1026 [M - Se + Na]+ (100%). 2′-(tert-Butyldimethylsilyl)-4-deacetyl-7-(triethylsilyl)5,20-deoxy-5,20-selenapaclitaxel (22). The reaction of selenium compound 20 (10.0 mg, 0.01 mmol) in THF (0.2 mL) with phenyllithium (30 µL, 0.05 mmoL, 5 equiv) at -78 °C for 3 min afforded compound 22 (5.2 mg, 43%): 1H NMR δ -0.30 (s, 3H), -0.10 (s, 3H), 0.60 (q, 6H, J ) 8.0 Hz), 0.80 (s, 9H), 0.83 (t, 9H, J ) 8.0 Hz), 1.18 (s, 3H), 1.20 (s, 3H), 1.60 (s, 3H), 2.18 (s, 3H), 2.20 (s, 3H), 2.38 (m, 1H), 2.60 (m, 2H), 2.70 (d, 1H, J ) 11.0 Hz), 3.05 (m, 1H), 3.10 (d, 1H, J ) 5.0 Hz), 3.30 (m, 1H), 3.82 (d, 1H, J ) 11.0 Hz), 3.95 (m, 1H), 4.52 (d, 1H, J ) 1.8 Hz), 5.05 (s, 1H), 5.60 (d, 1H, J ) 5.0 Hz), 5.90 (m, 1H), 6.15 (dd, 1H, J ) 1.7, 8.9 Hz), 6.49 (s, 1H), 7.20-7.60 (m, 11H), 7.80 (d, 2H, J ) 8.0 Hz), 8.18 (d, 2H, J ) 8.0 Hz); 13C NMR δ -5.8, -5.5, 5.3, 6.8, 10.9, 16.2, 18.3, 18.9, 20.9, 25.6, 27.3, 29.2, 35.5, 39.2, 41.3, 42.9, 54.7, 55.1, 60.4, 71.8, 72.8, 75.3, 75.4, 75.5, 77.3, 81.8, 126.9, 127.1, 128.3, 128.6, 128.8, 129.1, 130.4, 131.7, 133.3, 134.9, 135.9, 138.8, 139.6, 166.7, 167.6, 169.5, 170.3, 202.2; HRFABMS calcd for C57H77NO12Si2SeNa [M + Na]+ m/z 1126.4047, found 1126.4021. 4-Deacetyl-5,20-deoxy-5,20-selenapaclitaxel (23). Deprotection of the selena compound 22 (7.5 mg, 0.007 mmol) with hydrogen fluoride-pyridine (75 µL) yielded 23 (4.1 mg, 70%) as a colorless amorphous solid: 1H NMR δ 1.20 (s, 6H), 1.60 (s, 3H), 2.00 (s, 3H), 2.12 (s, 3H), 2.40 (m, 1H), 2.50 (m, 2H), 2.67 (d, 1H, J ) 10.0 Hz), 3.14 (d, 1H, J ) 5.0 Hz), 3.19 (m,

Novel Paclitaxol D-Ring Modified Analogues 1H), 3.28 (m, 1H), 3.38 (br s, 1H), 3.84 (m, 2H), 4.54 (br s, 1H), 5.30 (br s, 1H), 5.52 (d, 1H, J ) 5.0 Hz), 5.59 (m, 1H), 6.20 (dd, 1H, J ) 1.7, 8.9 Hz), 6.35 (s, 1H), 7.10-7.60 (m, 11H), 7.80 (d, 2H, J ) 8.0 Hz), 8.20 (d, 2H, J ) 8.0 Hz); 13C NMR δ 10.5, 16.6, 19.4, 20.9, 27.6, 29.7, 31.6, 35.2, 39.1, 42.8, 53.9, 60.0, 72.8, 73.5, 75.4, 75.7, 77.5, 81.9, 127.1, 127.2, 128.2, 128.7, 128.8, 129.0, 130.3, 131.8, 133.4, 134.5, 135.6, 139.8, 140.1, 166.8, 167.4, 170.9, 172.1, 204.2; HRFABMS calcd for C45H49NO12SeNa [M + Na]+ m/z 898.2318, found 898.2333.

Acknowledgment. We are grateful for financial support of this work by grant CA 69571 from the

J. Org. Chem., Vol. 64, No. 8, 1999 2703

National Cancer Institute to D.G.I.K. and by a grant from the Universidade Estadual de Maringa to M.H.S. The assistance of Bristol-Myers Squibb Pharmaceutical Research Institute in performing the HCT 116 cytotoxicity assay on compound 23 is also gratefully acknowledged. Supporting Information Available: 1H NMR spectra of compounds 8, 10, 11-20, 22, 23, and 25. This material is available free of charge via the Internet at http://pubs.acs.org. JO982095H